Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently colored, light-emitting elements commonly referred to as sub-pixels, typically emitting red, green, and blue light, to represent each image element. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode displays.
Light emitting diodes (LEDs) incorporating thin films of light-emitting materials forming light-emitting elements have, many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 issued May 7, 2002 to Tang et al. shows an organic LED (OLED) color display that includes an array of organic LED light-emitting elements. Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, charge transport, or charge blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a sub-pixel when current passes through the light-emitting material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
In order for light to be emitted from an LED device, at least one electrode is transparent. Transparent electrodes are typically formed from transparent conductive oxides such as indium tin oxide (ITO). However, transparent conductive oxides are especially problematic for flexible devices since they are brittle and subject to cracking when stressed. The cracking reduces the conductivity of the electrode and can degrade the light-emitting materials.
Two different methods for controlling the pixels in a flat-panel display device are generally known: active-matrix control and passive-matrix control. In an active-matrix device, control elements are distributed over the flat-panel substrate. Typically, each sub-pixel is controlled by one control element and each control element includes at least one transistor. For example, in a simple, prior-art active-matrix organic light-emitting (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each light-emitting element typically employs an independent control electrode and a common electrode.
One common, prior-art method of forming active-matrix control elements typically deposits thin films of semiconductor materials, such as silicon, onto a glass substrate and then forms the semiconductor materials into transistors and capacitors through photolithographic processes. The thin-film silicon can be either amorphous or polycrystalline. Thin-film transistors (TFTs) made from amorphous or polycrystalline silicon are relatively large and have lower performance compared to conventional transistors made in crystalline silicon wafers. Moreover, such thin-film devices typically exhibit local or large-area non-uniformity across the glass substrate that results in non-uniformity in the electrical performance and visual appearance of displays employing such materials. In flexible applications, the relatively large thin-film components are subject to considerable stress that modifies and degrades the performance of the thin-film components.
Matsumura et al describe crystalline silicon substrates used for driving LCD displays in US Patent Application No. 2006/0055864. The application describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown. The pixel control devices have a thickness of approximately 600 microns. Polishing techniques can be employed to reduce the thickness to approximately 200 microns. However, this thickness is still too large to effectively employ such pixel-control devices on a flexible substrate with a useful bending radius less than 2 cm.
Techniques for the reduction of stress in materials deposited on a substrate are described in “Theoretical and Experiment Studies of Bending of Inorganic Electronic Materials on Plastic Substrates” in Advanced Functional Materials 2008, 18, 2673-2684 by Park et al. This paper demonstrates that inorganic materials on flexible substrates have at least three failure modes: cracking, slipping, and delamination. The materials described and tested are not capable of controlling light-emitting elements in a display.
There is a need, therefore, for an improved flexible emissive device incorporating a chiplet.